Don't use a mask when not needed

When the mask has the same size as the parent register of a field, then
a mask is not required. This removes the clippy warnings/errors where
for example an operation of inverting a mask results in zero

```
error: this operation will always return zero. This is likely not the intended outcome
   |
61 |         self.w.bits = (self.w.bits & !0xffff_ffff) | (value as u32 & 0xffff_ffff);
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
```

or when the operation of using a mask is ineffective.

```
warning: the operation is ineffective. Consider reducing it to `value as u32`
   |
61 |         self.w.bits = (self.w.bits & !0xffff_ffff) | (value as u32 & 0xffff_ffff);
   |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
```

Author: thvdveld

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   elements.  An `ArrayProxy` wrapper is used when a Rust built-in array does not
   match the cluster layout.  Requires the `--const_generic` command line option.
 - Bumped `xtensa-lx` and add `xtensa_lx::interrupt::InterruptNumber` implementation.
+- Don't use a mask when the width of the mask is the same as the width of the parent register.
 
 ## [v0.19.0] - 2021-05-26
 

src/generate/register.rs

@@ -302,6 +302,12 @@ pub fn fields(
         let fty = width.to_ty()?;
         let evs = &f.enumerated_values;
 
+        let use_mask = if let Some(size) = parent.size {
+            size != width
+        } else {
+            false
+        };
+
         let lookup_results = lookup(
             evs,
             fields,
@@ -374,10 +380,14 @@ pub fn fields(
                 quote! {
                     ((self.bits >> #offset) & #hexmask) #cast
                 }
-            } else {
+            } else if use_mask {
                 quote! {
                     (self.bits & #hexmask) #cast
                 }
+            } else {
+                quote! {
+                    self.bits #cast
+                }
             };
 
             if let Some((first, dim, increment, suffixes, suffixes_str)) = &field_dim {
@@ -398,10 +408,14 @@ pub fn fields(
                         quote! {
                             ((self.bits >> #sub_offset) & #hexmask) #cast
                         }
-                    } else {
+                    } else if use_mask {
                         quote! {
                             (self.bits & #hexmask) #cast
                         }
+                    } else {
+                        quote! {
+                            self.bits #cast
+                        }
                     };
                     let name_sc_n = Ident::new(
                         &util::replace_suffix(&f.name.to_sanitized_snake_case(), suffix),
@@ -679,11 +693,17 @@ pub fn fields(
                         }
                     }
                 } else {
+                    let op = if use_mask {
+                        quote!{ self.w.bits = (self.w.bits & !#hexmask) | (value as #rty & #hexmask); }
+                    } else {
+                        quote!{ self.w.bits = value as #rty; }
+                    };
+
                     quote! {
                         ///Writes raw bits to the field
                         #inline
                         pub #unsafety fn #bits(self, value: #fty) -> &'a mut W {
-                            self.w.bits = (self.w.bits & !#hexmask) | (value as #rty & #hexmask);
+                            #op
                             self.w
                         }
                     }